Ca2+ activation of wheat peroxidase: A possible physiological mechanism of control

Peroxidation of substrates such as ascorbic acid, pyrogallol, or ferulic acid, as well as indole acetic acid oxidation catalyzed by wheat germ peroxidase (WGP)(2) C-2, were found to be activated by Ca2+. This activation is independent of the stabilizing effect of structural Ca2+ reported for peroxidases, Steady state kinetics of ferulic acid oxidation catalyzed by WGP Ca showed an increase in the rate of compound I formation and of compound II decomposition in the presence of the ion, evidenced as an increase in rate constants k(1), from 8.9 x 10(5) to 4.5 x 10(6) M(-1) cm(-1), and k(3), from 4.4 x 10(5) to 1.1 x 10(6) M(-1) cm(-1). The dissociation constant K-d, for the cyanide derivative of the enzyme showed a marked decrease from 220 to 34 mu M in the presence of Ca2+, thus implying an effect of the ion in the H2O2 binding step, In the presence of Ca2+, a conformational change in the protein was revealed by tryptophan fluorescence, providing a basis for the activation mechanism, Other peroxidases such as horseradish peroxidase and WGP C-3 were not activated by Ca2+, The results suggest the existence of a physiological mechanism of control of peroxidase isozymes activity mediated by Ca2+. (C) 1996 Academic Press, Inc.